Tropical storm Erika headed for eastern Caribbean

MIAMI (Reuters) - Tropical storm watches were issued early Tuesday as Erika, the fifth named storm of the 2015 Atlantic Hurricane Season, moved towards the Eastern Caribbean, the U.S. National Hurricane Center (NHC) said.

The tropical storm is likely to strengthen slowly, and could reach hurricane status over the Bahamas by the weekend, NHC said, though forecasters say its track is still unclear.

Erika formed on Monday and could pose a threat to the U.S. East Coast early next week if it manages to fight off dry air and wind shear, which are unfavorable for storm strengthening, according to Jeff Masters with the private forecaster, Weather Underground.

"However, Erika’s survival over the next few days is not a sure thing," he wrote in a blog post.

The storm was located about 730 miles (1,170 km) east of the Leeward Islands with maximum sustained winds of 45 miles per hour (75 kph). It was expected to near land by Wednesday night and early Thursday, the Miami-based hurricane center said in a Tuesday morning advisory.

Tropical storm watches were issued for the islands of Guadeloupe, St. Martin /St Maarten, St. Barthelemy, Montserrat, Antigua and Barbuda, St. Kitts and Nevis, Anguilla, Saba and St. Eustatius.

The government's annual forecast predicted a quieter-than-normal 2015 Atlantic hurricane season, with six to 10 named storms and up to four reaching hurricane status of 74 mph (119 kph).

Last week the season's first hurricane, Danny, while still far out at sea reached Category 3 on the Saffir-Simpson scale of intensity, with winds of between 111 and 129 mph (178-208 kph), before rapidly dissipating as it reached the Caribbean islands.

The Saffir-Simpson scale measures potential property damage from a storm, with hurricanes reaching Category 3 and higher considered "major" hurricanes with a potential for significant loss of life and damage.

Among the factors in this year's predicted weaker hurricane season is the El Niño weather phenomenon, the warming of Pacific waters that affects wind circulation patterns and makes the formation of hurricanes in the Atlantic-Caribbean basin less likely.

(Reporting by David Adams in Miami and Koustav Samanta in Bengaluru; Editing by Andrew Heavens and Alden Bentley)